Never, Ever smile when you apply blush, it ages you instantly, yet almost every women I know does it!

The reason you should keep a straight face is that when you smile
your create lots of tiny wrinkles, however young you are. The blush
cannot penetrate into these lines so when you relax your face you are
left with little lines that look just like wrinkles.

Also, when you smile the area that protrudes is not the best place to
put blush. It’s too close to your mouth on a relaxed face. Cheek colour
needs to be high and closer to your eyes than your mouth.

Look at a summer shot of a model or film star and you will see the sunkissed glow really high up on the cheekbones.

So always consciously relax all the muscles in your face before applying blusher. This can be quite hard to do but it's well worth it.